Effectiveness of Particulate Traps on Construction Site Engines: VERT Final Measurements

Introduction

In 1993, Germany specified more stringent limits for diesel particulate emissions (TRGS 554 [1]). A limit of 0.2 mg/m3 concentration in respiratory air, defined as EC+OC, was specified for occupational health reasons. These specifications were also adopted in Switzerland and Austria. Germany subsequently tightened the limit to 0.1 mg/m3 (EC). The American Conference of Governmental Industrial Hygienists recently proposed a TLV for diesel particulates of 0.05 mg/m3. These exposure limits cannot be attained using conventional methods, such as increased ventilation. Instead, the particulate emissions must be intercepted at the source. The particulate emissions of existing engines must be reduced to 1-2% of the raw emissions.

Extensive tunnel-projects in Switzerland prompted the joint project VERT (Verminderung der Emissionen von Realmaschinen im Tunnelbau) to evaluate pertinent measures. The following organizations jointly launched the project VERT at year-end 1993. The aim was to evaluate the possibility of curtailing emissions with after-treatment of exhaust-gases from existing engines:

Problem definition: How much particulate emissions do construction site diesel engines nowadays emit?
Solution quest: Practicable and cost effective solutions suitable for retro-fitting the majority of construction site diesel engines currently field deployed.
Solution testing: Demonstrate in a field test during at least one year and more than 1,000 operating hours.
Enforcement: Find tamper-proof methods and instruments for easy periodic verification under field conditions.

The results of the 4-year investigations of construction site engines on test rigs and in the field are clear: particulate trap technology is the only acceptable choice among all available measures. Traps proved to be an extremely efficient method to curtail the finest particles. Several systems demonstrated a filtration rate of more than 99% for ultra-fine particulates. Specific development may further improve the filtration rate.

A two-year field test, with subsequent trap inspection, confirmed the results pertaining to filtration characteristics of ultra-fine particles. No curtailment of the ultra-fine particulates is obtained with any of the following: reformulated fuel, new lubricants, oxidation catalytic converters, and optimization of the engine combustion [2].

Particulate traps represent the best available technology (BAT). Traps must therefore be employed to curtail the particulate emissions that the law demands are minimized. This technology was implemented in occupational health programs in Germany, Switzerland and Austria.

Engine Bench Tests

29 particulate trap systems utilizing the following filter media and regeneration systems were investigated during the project.

Filter media

Ceramic monolithic cell filter (CORNING, NGK)

Metal sintered filter in cellular structure (SHW, HJS)

Fiber wound filter (3M)

Fiber woven filter (HUG)

Fiber knitted filter (BUCK, OBERLAND)

Regeneration systems

Full flow diesel burner (DEUTZ)

Periodical electric regeneration (UNIKAT, HUSS)

Fuel additives (Fe-PLUTO, Cu-LUBRIZOL, Ce-RHODIA)

Catalytic coating (BUCK, HUG)

The investigations on the engine test rig examined the filtration characteristic under steady and transient conditions, deposition behavior and regeneration response. The main effort was devoted to the filtration characteristic of ultra-fine particulates under steady state conditions.

All steady-state measurements were performed as per ISO 8178 C1 at four operating points. These are at 100% and 50% load; each at rated RPM and the RPM of maximum torque. The transient conditions were based on free acceleration. Filter deposition and regeneration were investigated in a temperature step test.

The following results were obtained from the tested traps during the first run of the LIEBHERR 914 engines in 1994/95 [3]:

During these preliminary tests, the collected ultra-fine particulates were not dependably distinguished between solid particulates and condensates formed in the dilution tunnel. Thus, the sintered metal filter had the worst particulate count performance of 86% in the above table. This was clarified during later tests by separating solid particulates from condensates in the size analysis. The filtration rate of the sintered metal trap for solid ultrafine particles was 99% and more, despite the fact that this trap type is not the best filter available today. It is a typical surface filter. Therefore, it tends to a deterioration of the filtration rate for smaller particulates. In contrast, deep bed filters have the opposite characteristic. They can easily attain 99.9% filtration in the lower size range.

Previous gravimetric particulate criteria show a filtration rate of 70-80% for certain traps. Better values are obtained for the solid ultra-fine particulates. Under hot exhaust gas conditions, only the solid particulates can be dependably retained in traps. This finding was verified for several trap systems, within the scope of the VERT suitability tests [4, 5].

Another very positive feature of particulate traps is their PAH (polycyclic aromatic hydrocarbons) performance. It was found that the total PAHs decrease proportionally to the gravimetric decrease of carbon mass.

The above properties are extraordinarily positive and convincing. To complete the picture, the following possible risks of deploying particulate traps were examined for existing engines.

Back pressure

The trap inevitably increases the back-pressure. It is usually impossible to restrict this back pressure, during the entire operating period, to values comparable with exhaust mufflers. The negative consequences are increased pumping work, and thus a proportional deterioration of fuel consumption. Particularly for highly supercharged engines, an exaggerated deterioration of combustion can occur at back-pressures higher than 200 - 250 mbar. This again causes more raw emissions [6].

Therefore, the VERT criteria specify that the back-pressure of the new filter shall not exceed 50 mbar. Higher back pressures can periodically result from soot deposition. Another cause is gradual deterioration through ash accumulated during the useful filter life. This is carefully monitored through electronic measurement of the trap back-pressure. Thus, the maximum back-pressure is limited to 200 mbar.

Additive ash particulates

Fuel additives are employed to lower the soot ignition temperature. They usually consist of metals (Iron, Magnesium, Copper) or rare-earths (Cerium). These reappear as tiny oxide clusters in the exhaust gas. Measurements have shown that these particulates are substantially smaller than the conventional soot particulates, which results in a typical bimodal distribution as shown in Fig. 1 in a log-scale.

Therefore, fuel additives should only be used together with traps that can dependably filter ultra-fine particulates. This aspect was intensively investigated and substantiated [4]. The results are again reproduced in the Fig. 2.

Obviously, additives decrease the soot particulates in raw gas (15-25% by mass). The clean gas with the combination of trap + additive shows a very low level of carbonaceous particles around 100nm. The Fig. 2 also clearly indicates the secondary emissions from the additive ash particulates below 30 nm. These decrease considerably when the dosage is reduced. An additive dosage of 36 ppm, as used here, is at least twice above normal and was only used to clearly demonstrate the phenomena.

Further investigations indicate that a smaller dosage is sufficient. This is highly desirable to retard accumulation of additive ash in the trap.

De-novo synthesis of dioxins and furanes

It is known that at temperatures around 400°C dioxins and furanes can be synthesized in the exhaust gas. The pre-requisites are a sufficient dwell time and the presence of chlorine, hydrocarbons and soot. Certain metals, particularly copper, catalytically promote the synthesis of these toxic substances.

These prerequisites are prevalent in the particulate trap system:

Chlorine in fuel up to 7 ppm, in lube oil up to 300 ppm

Temperature aperture 300-450°C is frequent

HC and soot are available plentifully

High dwell times particularly occur when the reaction gases are adsorbed onto the large surface of the deposited soot (200 m2/g) in the particulate trap. Thus, they can dwell for a long time under these conditions.

Additional metallic fuel additives could cause an undesirable acceleration of the synthesis. This hazard must be clarified before the particulate trap technology, particularly the pertinent additives, is permitted. An extensive investigation launched within the scope of the VERT project was concluded in 1997. The results are shown in Fig. 3.

The baseline using the reference fuel, without chlorination, is at 30pg PCDD/F per liter fuel. Clearly, chlorinating does not amplify de-novo synthesis of dioxins and furanes in the trap, neither with the iron additive nor with the cerium additive. Adding copper immediately increases these emissions. Copper reveals the very pronounced function of the particulate trap as an adsorptive reactor. If the chlorine content is increased to a slightly higher (but still realistic) level, then the dioxins and furanes emissions increase by 3 orders of magnitude.

The results are only a partial extract from a very comprehensive investigation [7]. Elevated dioxin and furane emissions attributed to the presence of copper were also reported by Australian [8] and German researchers [9].

Copper additives must therefore be excluded. There is no objection to iron and cerium additives. Other additives must be investigated accordingly before deployment.

Field Tests

Table 1 list 10 systems that were field-tested.

Table 1: Particulate traps in the VERT field test

Engine

Filter supplier

Regeneration

Symbol

Liebherr

D904T

SHW(HJS)

Additive Eolys (Ce)

LIB1

D904T

BUCK

Catalytic coating

LIB2

D914TI

ECS

Additive Lubrizol (Cu)

LIB3

D916T

DEUTZ

Full flow diesel burner

LIB4

Caterpillar

3306TA

SHW

Additive satacen (Fe)

CAT1

3306T

DEUTZ

Full flow diesel burner

CAT2

3116T

BUCK

Additive satacen (Fe)

CAT3

3118

UNIKAT

Periodic electric

CAT4

3406T

UNIKAT

Periodic electric

CAT5

3116T

HUG

Catalytic coating

CAT6

The most important data, e.g. trap pressure loss and exhaust gas temperature, were acquired during the entire test period. Data was sampled every 2 -8 seconds and recorded with an on-board logger. Comprehensive exhaust gas measurements were performed in the field, at intervals of about 4-6 weeks.

Very valuable experience was thus accumulated during the test period. The filter CAT4 was deployed longest. Some representative results, for this filter, are shown below.

The periodic exhaust gas emission measurements were performed at low idling, high idling and full load. The average values give a good picture of the influence of this trap on the exhaust-gas. No negative aspects are detectable. The positive effect on CO and HC can be attributed to the supplementary catalytic coating of this filter. The slight NOx decrease is due to the exhaust gas retention caused by trap back-pressure. Most traps show a very similar picture. The decrease of CO and HC also occurs in uncoated filters, particularly when they are operated with regeneration additives. An abnormal influence on the exhaust gas emission was never observed through the trap function.

The VERT report [5] describes these extensive field tests. Some observations are reproduced here.

Mounting the filter was usually trouble-free. Often, the trap could be mounted in place of the muffler and had the same dimensions. Thus, rapid replacement was possible. The muffling effect was often even better.

All traps endured these long and arduous tests without serious damage.

The traps retained their ability to dependably filter particulates, in most cases, during the entire test period. However, both fiber traps suffered saturation phenomena due to insufficient regeneration (cases of very low exhaust gas temperatures). Therefore, the back pressure reached the limiting value and the filtration rate sank. After regeneration, the traps recovered their good initial values.

Regeneration with fuel additives was successful in most cases. The additives should be automatically dosed on-board. The useful period until filter cleaning should be 2000 hours, but was not attained in field tests.

An intelligent electronic monitoring of trap back pressure is necessary. Data logging and alarm functions should also be included. The monitoring must be tamper-proof and indicate the upper limit (clogging) and a lower limit (trap damage).

Altogether, these field tests proved that a whole series of trap systems have a high filtration rate and stable long time properties. They are capable of performing under difficult construction site conditions.

Remaining objectives in the further development of these systems are: (1) fully automatic self-controlled operation and regeneration with little or no maintenance, and (2) long life of the entire filter system under rough conditions.

Evaluation After Field Tests

Mass Filtration Efficiency

The field tests lasted two years and accumulated between 1,500 and 7,000 operating hours. Afterwards, seven of the ten traps were re-tested on the engine rigs. Their filtration characteristic was investigated. The full range of particulate analysis instrumentation was employed. In particular the filtration rate was compared according to the total particulate mass and according to the particulate count.

Table 2 is an overview of average values from all four operating points:

Table 2: Filtration rate in % according to mass (PMAG) and count (PZAG) average at four points ISO 8178 for the trap in states as delivered "Standard diesel" and regeneration promoted with Fe additive

PMAG

PZAG

Standard diesel

Fe additive

Standard diesel

Fe additive

LIB2

76.52

81.00

95.38

97.80

LIB4

70.46

76.08

86.65

91.63

CAT1

77.54

87.64

97.79

98.80

CAT3

64.20

76.74

91.03

96.78

CAT4

54.76

76.54

98.98

99.60

LIB1

3.2

22.2

96.3

97.1

LIB3

12.4

43.0

99.9

ca. 99

The results are rather surprising. All traps show very good values for the concentration count. However, the filtration rate is sometimes very low according to the mass criterion. Particularly striking are the results for the two last listed traps LIB1 and LIB3, Both possess excellent filtration rates for the particulate count, but are completely unsatisfactory for the mass criterion.

Table 3 compares both filters at two operating points, i.e. full load and 50% load at rated RPM.

Table 3: Filtration rates in % according to mass (PMAG) and counts (PZAG) at full load and part load conditions

PMAG

PZAG

Standard diesel

Fe additive

Standard diesel

Fe additive

LIB1

Full loadPart load

-182.087.2

-67.889.7

93.797.6

97.198.0

LIB3

Full loadPart load

-80.093.0

-64.090.1

91.899.7

ca. 98ca. 99

Both traps suffer negative values for the mass criterion at full load, whereas the particulate count filtration-rate is very high. At part load, there is more uniformity in the values. The bad full-load values had a serious negative influence on the averages of Table 2. An analysis of the test filter explained the paradox at full load. Both filters had extremely high sulfate emissions that contributed heavily to the particulate mass. A more precise analysis revealed the differences between these two traps and the other five:

The LIB1 trap was badly clogged with additive ash at the end of the field test. Even after regeneration, the pressure loss was 400 mbar. Hence, the engine exhaust gas temperature was nearly 100°C higher than for traps with normal back pressure (<100 mbar). Higher exhaust gas temperatures result in more SO3 formation and therefore to sulfate deposition in the test filter.

The LIB3 trap was operated with copper additive. At the end of the field test, it only had a back pressure of 150 mbar, although it was only once cleaned. The exhaust gas temperature was only slightly above normal. Probably, the catalytic activity of the copper oxide is here responsible for the increased SO3 formation detected in the analysis of the filter residue.

The precursor substances of the sulfate are gaseous at the exhaust gas temperatures flowing through the filter medium. The engine essentially emits SO2. This is converted to SO3 at the increased temperatures or through catalytic promotion. Sulfuric acid is first formed during the cooling in the test tunnel with the always plentiful water. This acid, together with additional azeotropic water binding, influences the mass of test filter residue.

Fig. 6: Falsified poor filtration rate because of high sulfate formation at full load temperatures

Both examples dramatically demonstrate the unsuitability of gravimetric criterion in the context of the current legally prescribed tunnel measurement technology. The unrealistically low dilution is inappropriate for the evaluation of filter media.

Particulate Count

After the completion of field tests, the traps were also tested for their filtration characteristics at different particle sizes. The tests were performed both with standard diesel fuel (no additive) and with fuel additives. Six out of the seven tested traps attained filtration efficiency of 99% or more for the ultra-fine particulates. The following chart illustrates particle size distribution with and without the CAT4 trap. That trap experienced the longest deployment period in the field of more than 7000 hrs. The field tests started at 5715 hrs.

Fig. 7: Size distribution of solid particles with/without CAT4 trap, as also with/without additive at full load and rated RPM

The following bar chart (Fig. 8) is an overview of the CAT4 trap performance at different engine modes.

Fig. 8: Concentration count integrated in the range 20 - 200 nm

This representation integrates the particulate count for all size ranges from 20 - 200 nm. The result is compared against the cases with/without filter, with/without additive at all 4 operating points. The picture is very uniform. Convincingly, when using traps, the particulate concentration count diminished at least two orders of magnitude, in all cases.

Elemental Carbon

The original objective of the German occupational health regulations (TRGS 554 [1]) was neither the particulate count nor its total mass. Rather, the criteria are the mass of elementary carbon (EC) and total carbon (TC). The TC analysis requires a complex coulometric method. This method has not been used in this investigation at all operating points. As a rule, only the full load point at partial RPM has been examined.

This evaluation shows efficiency based on gravimetric results for elementary carbon EC and total carbon TC. Both values are equally good and comparable with the evaluation of concentration count of solid particulates. Hence, the hypothesis that the solid particulates are a composite of an elementary carbon core onto which hydrocarbons are deposited. Therefore, both methods deliver comparable results, albeit the particulate count is much more sensitive, more accurate, and easier to perform.

Fig. 9 shows distinctly the discrepancy with the evaluation according to total particulate mass, which is inappropriate for the evaluation of these hot gas particulate traps.

Fig. 9: CAT4 filter function according to Coulometry

The trap demonstrates comparably good filtration rates also under transient conditions. Filters after long periods of field deployment were still able to reduce diesel smoke opacity from about 90% to well below 10% during free acceleration test [10].

The opacity measurement, however, does not correctly reproduce the ability of the trap to filter ultra-fine particulates. Like the conventional tunnel CVS measurement technique, it is also susceptible to condensate formation because of abrupt cooling without sufficient dilution. Only the newer sensor technology [4] facilitates particulate counting during transients. The results confirm that filtration rates of 99% and more can be attained under transients, too.

Conclusions

Traps are effective in the entire size range of diesel particulates, starting from primary particulates of about 10 nm.

Traps can be deployed for new engines. They can also be retrofitted to existing older engines. Hence, they are suitable for rapid and wide-spread application to reduce the particulate matter exposure.

Several methods are suitable for regenerating particulate traps.

This technology does not produce secondary emissions.

Fuel additives can be recommended in combination with appropriate traps [11].

Development of nano-particulate measurement techniques suitable for field deployment and for large scale industrial application [12].

As a result of the VERT studies, it is now required by German, Swiss and Austrian law that filters be installed in diesel powered tunnelling equipment.

Acknowledgements

The authors thankfully acknowledge publishing granted by the VERT sponsors: Suva, AUVA, TBG and BUWAL. They are grateful to the participating laboratories for the thorough and stimulating collaboration, as well as the industrial partners for advice and help during the entire project.